(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)
Recently, Hawaiian Volcano Observatory scientist’s have been asked if Hawaii Island’s geothermal development causes subsidence of the ground surface, as has been observed at U.S. mainland geothermal areas, such as those in California– Coso, Geysers, the Imperial Valley, and Casa Diablo in the Mammoth Lakes area.
The largest of these mainland developments is heated by magma reservoirs that supplied eruptions through non-volcanic rock layers. The magma, thousands of years old, still heats groundwater within California’s sedimentary basins, where, fluid withdrawal during geothermal energy production can reduce pressure within the subsurface rock formations. When this happens, the ground surface subsides.
The highest subsidence rates are measured at the Geysers Geothermal Field within the Clear Lake Volcanic Field in northern California. The 78 square kilometer (30 sq mi) developed area produces 1,500 megawatts (MW) of electricity from a vapor-dominated reservoir within fractured sandstone that is capped by a zone of rock filled with geothermal minerals at the top and heated by magma below.
The Clear Lake volcanic field is one of several in California and Nevada that are monitored by HVO’s sister California Volcano Observatory (CalVO; http://volcanoes.usgs.gov/observatories/calvo/). CalVO also monitors the Long Valley caldera, in which the Casa Diablo geothermal development (40 MW) is located; the Salton Buttes volcano, which includes the Imperial Valley geothermal field (>300 MW); and the Coso Volcanic Field, where a 270 MW geothermal development is located.
As volcanic activity waxes and wanes, each of these areas exhibits its own deformation and seismicity. For example, the Long Valley caldera has experienced several episodes of heightened unrest, including earthquake swarms, ground uplift, and volcanic gas emissions during the past several decades. Thus, CalVO watches this area closely. The subsidence related to geothermal development must be documented carefully to separate the effects of volcanic activity from those due to drilling and energy production.
In Hawai`i, the most attractive geothermal resource is within Kilauea volcano’s active east rift zone. Volcanic rift zones—linear zones along which volcanic eruptions occur—are areas that include highest hazards for lava flows, volcanic gases, and tephra (cinders and ash) from an eruption.
Rift zones also spread apart and subside to accommodate the magma injected within it, which can be hazardous if the subsidence happens suddenly. Another way in which rift zones can become active was demonstrated by the November 1975 earthquake. During that quake, the lower east rift zone abruptly dropped about 25 cm (10 in).Since that time, HVO has documented subsidence of the lower east rift zone at an average rate of about 1 cm (0.4 in) per year.
If there is any subsidence related to Puna Geothermal Venture (PGV) energy production, it is much less than the natural subsidence of Kilauea’s east rift zone. It is also less than the subsidence observed at the California geothermal fields.
The differences in subsidence may be due to the geologic nature of the geothermal production zones or the properties of the hydrothermal reservoir. In Hawai`i, the producing zones are entirely volcanic rock, through which the hydrothermal fluid moves along fractures. In California, the producing zones are in layers of sediment or other non-volcanic rocks.
There are also large differences in the total percentage of fluids extracted and re-injected into the production zones. In Hawai`i, PGV reports that all fluids and gases pumped out are re-injected deep into the volcanic rock reservoir. In California’s Geysers geothermal field, less than half the fluids pumped out are injected back into the reservoir.
To further contrast with mainland geothermal fields, the chance of a volcanic eruption affecting Kilauea Volcano’s east rift geothermal plants is far greater than that for any of California’s geothermal fields. As HVO continues to monitor Kilauea’s lower east rift zone for signs of future volcanic activity, the natural subsidence rate of 1 cm/yr (0.4 in/yr) is only one of the signs or indicators we watch. Thus, it is necessary to separate the effects of natural subsurface volcanic processes from other human-made processes, such as geothermal energy production.